Despite the well-recognized importance of surface hydrophobicity in many aspects of nanotechnologies, existing methods (e.g., dye sorption assays and hydrophobic interaction chromatograph) designed to measure or compare hydrophobicity have significant limitations when applied to nanomaterials (NMs). Determining NM’s hydrophobicity is important for improving the prediction of NM’s fate and transport in the environment. The aim of this work is to apply a novel alternative method in directly characterizing the changes in NM hydrophobicity induced by sulfidation and acquisition of natural organic matter corona in aqueous systems. Using dark field microscopy, this method quantitatively determines the surface energy components of NMs by measuring their adsorption kinetics to a set of surfaces with tuned hydrophobicity. The total surface energy of a NM is the sum of the Lifschitz-van der Waals (γLW, non-polar component) and of the acid-base component (γAB, polar component). An increase in a NM’s γAB corresponds to an increase of its hydrophilicity. In this study, we characterized 50 nm silver nanoparticles (AgNPs) functionalized with either polyethylene glycol (PEG) or polyvinyl pyrrolidone (PVP). We sulfidized each type of AgNPs with a range of S/Ag ratio to model “environmentally transformed” NPs under reducing aquatic conditions. Surface hydrophobicity of pristine and sulfidized AgNPs were determined before and after exposure to natural organic matter (NOM). We hypothesized that 1) pristine AgNPs become more hydrophobic upon sulfidation due to the detachment of ligands (PEG/PVP) and/or the formation of a sulfide layer on the particles’ surface, 2) sulfidized AgNPs become less hydrophobic upon acquisition of a NOM corona with oxygen-containing functional groups. Our preliminary results reported a decrease in the polar component of the surface energy (γAB) of PEG-AgNPs from &gt;6 mJ/m2 to 2.91 mJ/m2, indicating that PEG-AgNPs became more hydrophobic after sulfidation (S/Ag = 1.5). This shift corresponded to an increase in equivalent calculated contact angle from &lt;20° to 70°. Upon acquisition of a NOM corona, sulfidized PEG-AgNPs (S/Ag = 1.1) were found to become more hydrophilic (γAB shifted from 3.73 mJ/m2 to 6 mJ/m2). Overall, these initial findings support our hypotheses. More experiments are currently underway to confirm these results and to examine other experimental conditions.

Quantification of changes in nanoparticle hydrophobicity induced by sulfidation and acquisition of natural organic matter corona

ACS Spring Meeting 2021

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